Photolithography is a common technique employed in the manufacture of semiconductor devices. Typically, a semiconductor wafer is coated with a layer of light sensitive resist material (photoresist). Using a patterned mask or reticle, the wafer is exposed to projected light, typically actinic light, which manifests a photochemical effect on the photoresist, which is ultimately chemically etched away, leaving a pattern of photoresist "lines" on the wafer corresponding to the pattern on the mask or reticle. A simplified diagram of a photolithographic apparatus is illustrated in FIG. 1, discussed hereinafter.
One type of mask used in the photolithography process is a chromed glass or quartz plate bearing the pattern to be projected onto the photoresist layer. Light is projected through the mask, and those areas of the mask which are not chromed allow the light to expose the photoresist, while those areas of the mask which are chromed prevent the light from exposing the photoresist. The exposed areas of the photoresist resist chemical etching.
A typical photolithography apparatus is the Ultratech Stepper Model 900 projection stepper. A projection stepper is a device that presents a sequence of masks (on a reticle) between the light source and the wafer.
Commonly-owned U.S. Pat. No. 4,652,134, entitled MASK ALIGNMENT SYSTEM, discloses a system for aligning a semiconductor wafer with a mask bearing a pattern to be formed on the wafer, and is incorporated by reference herein.
The major causes of nonuniformity of line widths in semiconductor wafers are nonuniformity of illumination, and "intrinsic" materials qualities having to do with contrast. The illumination uniformity of photolithographic apparatus will often set a limit to how small a feature, such as a line, can be imaged in a manufacturing environment. Current systems have illumination uniformities of plus or minus 1-3%. In other words, illumination nonuniformity represents approximately a 6% error source, an overwhelming amount when one is designing against an "error budget" that may only be 10%. This is a very significant excursion for a very high contrast material. Furthermore, these illumination error figures tend to be optimistic, and represent what the bulb manufacturer obtained under ideal conditions. Ultimately, illumination nonuniformity establishes a lower limit on any line width obtainable using photolithographic techniques, consequently limiting the number of devices that can practically reside on a wafer of given size.
Various techniques have been proposed and practiced for increasing photolithographic illumination uniformity. These include the use of uniformizers (light homogenizers), such as "fly's-eye" lenses, light pipes and fiber optic bundles. Fly's-eye lenses, for example, create an overlapping matrix of images of a light source. Light pipes and fiber optic bundles rely on internal reflections to uniformize the light from the light source. However, these techniques appear to have reached a limit to their effectiveness, repeatability and practicality.